Assessing the Threat of Oort Cloud Comet Showers Nathan Kaib & Tom Quinn University of Washington

Outline Long-Period Comet Production Jupiter-Saturn Barrier Simulation Results Comet Shower Probability

Long-Period Comets

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Jupiter-Saturn Barrier Comets must have large perihelion shift to make it past Jupiter/Saturn in one orbital period Only weakly bound comets will have large perihelion changes Jupiter/Saturn shield inner solar system from inner 20,000 AU of Oort Cloud AU

Comet Showers AU Rare close stellar encounters (< 5000 AU) are able to perturb more tightly bound orbits The Earth is temporarily exposed to the entire Oort Cloud

Simulations Initial cloud orbits (~10 6 ) drawn from recent OC formation simulation results (Kaib & Quinn, accepted) Modify SWIFT (Levison & Duncan, 1994) with time-reversible adaptive timestepping routine (Kaib & Quinn, accepted) Evolved under influence of Sun, 4 giant planets, Milky Way tide and passing stars Vary stellar mass, impact parameter, and encounter velocity

M * = M Sun v  = 20 km/s, D min = 3000 AU  t = 10 5 yrs 25,000 AU4 AU

Quantifying Shower Strength LPC defined as q < 5 AU M * = 0.8 M Sun v  = 20 km/s D min = 1300 AU Calculate years for normal LPC flux to produce comets

v  = 20 km/s Simulation Results

Use impulse approximation to calculate  v Sun for each stellar passage:  v Sun = (2GM * )/(bv  )

One parameter controls shower strength

Finding Shower Frequency Use Rickman et al. (accepted) stellar encounter code to generate ~10 6 passages Find dN(  v Sun )/dt

1/  ~ (  v Sun ) -2 (Rickman et al., accepted)

Conclusions Comet shower intensity for a given Oort Cloud is basically governed by one parameter:  v Sun Showers releasing at least 10 Myrs of LPCs occur every 100 Myrs Showers releasing at least 40 Myrs occur every Gyr

Conclusions II Transforming results to an impact rate is HARD: Need to know real current flux of LPCs Weissman (2007) estimates 1 background LPC impact every 38 Myrs Implies 1 shower-induced impact every Gyr Conservative estimate: we use a minimum inner Oort Cloud population

Shower Semimajor Axes

Divide LPC distribution by Oort Cloud distribution  Probability of LPC as a function of a

Regions Sampled by LPCs

Effects of Solar Formation Setting Inner Oort Cloud population very sensitive to formation environment of Sun (Fernadez & Brunini, 2000; Brasser et al., 2006; Kaib & Quinn, accepted)

r = 300 AU r = 400 AU  t = 200 days  t = 10 yrs  t = 50 yrs Not Symplectic: q drifts by ~0.4 AU over sim

Building an Oort Cloud

The tide of the Milky Way also perturbs the Oort Cloud (COBE, NASA)

Galactic tide causes perihelion and inclination to oscillateAbout 2x as powerful as stellar passages (Heisler & Tremaine 1986)